Jackalyne Pfannenstiel, the assistant secretary of the Navy for energy, installations and environment, announced her resignation on Friday for unspecified health reasons, the service said.

In her position, Pfannenstiel’s has been overseeing Navy Secretary Ray Mabus’ initiative to expand the use of alternative and bio-fuels in the Navy surface and aviation fleet, a plan that has drawn criticism form some members of Congress who view it as too costly or believe the Navy should be more focused on warfighting and shipbuilding than developing alternative energy.

“This has been one of the most challenging and rewarding positions of my career,” Pfannenstiel said in a statement released by the Navy. “The Navy and Marine Corps are pushing the limits of what renewable energy can do for military readiness, and I’ve been privileged to be a part of that.”

Pfannenstiel has served in the position for two years and came from the California Energy Commission. Mabus sought Pfannenstiel to carry out his plan of having the Navy use 50 percent of alternative fuels by 2020. Her resignation is effective July 1.

The Senate Armed Services Committee fiscal year 2013 defense authorization bill contains language largely prohibiting the Pentagon from buying alternative fuels if they cost more than traditional fossil fuels and also bans the Pentagon from building a bio-fuels refinery unless specifically authorized by law (Defense Daily, May 29).